Cathode for secondary battery

ABSTRACT

Disclosed is a cathode for secondary batteries comprising a compound having a transition metal layer containing lithium as at least one compound selected from the following formula 1: (1−x)Li(Li y M 1-y-z Ma z )O 2-b A b *xLi 3 PO 4  (1) wherein M is an element stable for a six-coordination structure, which is at least one selected from transition metals that belong to first and second period elements; Ma is a metal or non-metal element stable for a six-coordination structure; A is at least one selected from the group consisting of halogen, sulfur, chalcogenide compounds and nitrogen; 0&lt;x&lt;0.1; 0&lt;y&lt;0.3; 0≦z&lt;0.2; and 0≦b&lt;0.1.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT International Application No.PCT/KR2011/003063 filed on Apr. 27, 2011, which claims priority under 35U.S.C. §119(a) to Patent Application No. 10-2010-0041030 filed in theRepublic of Korea on Apr. 30, 2010, all of which are hereby expresslyincorporated by reference into the present application.

TECHNICAL FIELD

The present invention relates to a cathode for secondary batteries. Morespecifically, the present invention relates to a cathode for secondarybatteries having long lifespan and superior storage properties andexerting superior safety based on a specific element composition.

BACKGROUND ART

Technological development and increased demand for mobile equipment haveled to a rapid increase in the demand for secondary batteries as energysources. Among these secondary batteries, lithium secondary batterieshaving high energy density and voltage, long lifespan and lowself-discharge are commercially available and widely used.

In addition, increased interest in environmental issues has broughtabout a great deal of research associated with electric vehicles (EVs)and hybrid electric vehicles (HEVs) as substitutes for vehicles usingfossil fuels such as gasoline vehicles and diesel vehicles which aremain factors of air pollution. These electric vehicles generally usenickel hydride metal (Ni-MH) secondary batteries as power sources ofwith electric vehicles (EVs), hybrid electric vehicles (HEVs) and thelike. However, a great deal of study associated with use of lithiumsecondary batteries with high energy density and discharge voltage iscurrently underway and some of them are commercially available.

In particular, lithium secondary batteries used for electric vehiclesshould have high energy density, exhibit great power within a short timeand be used under harsh conditions for 10 years or longer, thusrequiring considerably superior stability and long lifespan, as comparedto conventional small lithium secondary batteries.

Conventional lithium secondary batteries generally utilize a lithiumcobalt composite oxide having a layered structure for a cathode and agraphite-based material for an anode. However, such lithium cobaltcomposite oxide is disadvantageously unsuitable for electric vehicles interms of presence of extremely expensive cobalt as a main element andlow safety. Accordingly, lithium manganese composite oxide having aspinel structure containing manganese that is cheap and has superiorsafety is suitable for use as a cathode of lithium ion secondarybatteries for electric vehicles.

However, lithium manganese composite oxides cause deterioration inbattery properties since manganese is released into an electrolyte dueto affection of the electrolyte when stored at high temperature.Accordingly, there is a need for a solution to this phenomenon. Inaddition, as compared to conventional lithium cobalt composite oxide orlithium nickel composite oxide, lithium manganese composite oxides havea disadvantage of low capacity per unit weight, thus having a limitationof an increase in capacity per battery weight. Lithium manganesecomposite oxide should be used in combination with battery designcapable of solving this phenomenon in order to allow the same to bepractically available as a power source of electric vehicles.

In order to solve these disadvantages, layered mixed metal oxides,LiNi_(x)Mn_(y)Co_(z)O₂ (x+y+z=1) and the like are used, but they cannotsecure satisfactory stability yet. Surface-treatment is attempted inorder to solve this disadvantage, but problems such as increase in pricewhich is one of the most important problems in the battery market suchas electric vehicles occur due to the necessity of additional processes.

DISCLOSURE Technical Problem

Therefore, the present invention has been made to solve the aboveproblems and other technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies andexperiments to solve the problems, as described below, the inventors ofthe present invention have discovered that, when a secondary battery isfabricated using a cathode comprising a cathode active material that hasa specific element composition as shown in the compound of Formula 1 andincludes a transition metal layer containing lithium, lifespan can begreatly improved without using additional processes. Based on thisdiscovery, the present invention has been completed.

Technical Solution

In accordance with one aspect of the present invention, provided is acathode for secondary batteries, comprising a compound having atransition metal layer containing lithium as at least one compoundselected from the following formula 1:(1−x)Li(Li_(y)M_(1-y-z)Ma_(z))O_(2-b)A_(b)*xLi₃PO₄  (1)

wherein M is an element stable for a six-coordination structure, whichis at least one selected from transition metals that belong to first andsecond period elements;

Ma is a metal or non-metal element stable for a six-coordinationstructure;

A is at least one selected from the group consisting of halogen, sulfur,chalcogenide compounds and nitrogen;0<x<0.1;0<y<0.3;0≦z<0.2;and0≦b<0.1.

The cathode according to the present invention realizes stabilization ofcrystal structure caused by variation in oxidation number by Li(lithium) present in the transition metal layer based on theaforementioned specific composition and exhibits improved cycleproperties and storage properties of an active material, since Li₃PO₄having a strong bonding force and ion conductivity is present on thesurface of active material particles or inside the same.

When the content of Li₃PO₄ is excessively high, it inhibitscrystallization of the cathode active material and it may be difficultto improve performance of the active material. As defined above, thecontent of Li₃PO₄ is preferably lower than 0.1, more preferably0<x≦0.05.

In addition, when the content of Li in the transition metal layer isexcessively high, deterioration in capacity may be caused. Accordingly,the defined content range is preferred and 0.01≦y≦0.2 is more preferred.

M of Formula 1 is a transition metal that satisfies the aforementionedconditions and may be one or more selected from Ni, Mn, Co, Cr, Fe, V,Zr and the like and the cathode active material preferably contains Niand Mn as essential elements and contains Co as an optional element. Inthis case, the contents of Ni, Mn and Co satisfy the equation of thefollowing formula 2.M=Ni_(a)Mn_(b)Co_(c)  (2)

wherein 0.10<a<0.85, 0.10<b<0.85, and 0≦c<0.5.

As defined above, the transition metal (M) may be substituted at anamount defined by A of Formula 1 by a metal or non-metal element havinga six-coordination structure such as Al, Mg or B.

In addition, oxygen (O) may be also substituted at an amount defined inFormula 1 by other anions, for example, halogen elements such as F, Cl,Br and I, sulfur, chalcogenide compounds, nitrogen and the like.

The compound of Formula 1 may be prepared by a method well-known in theart, based on the composition of the formula.

The cathode active material may be for example prepared by preparing amixed transition metal precursor by a variety of methods such asco-precipitation, adding a lithium compound such as lithium hydroxideand lithium carbonate and lithium phosphate thereto and baking theresulting compound.

The cathode may contain a general lithium transition metal oxide thatdoes not satisfy the aforementioned conditions, as a cathode activematerial, in addition to the compound of Formula 1. The general lithiumtransition metal oxide includes oxides containing one of Ni, Co and Mnand oxides containing two or more thereof and examples thereof includelithium transition metal oxides known in the art. In this case, thecompound of Formula 1 may be present in an amount of at least 30% byweight or higher, preferably, 50% by weight or higher, based on thetotal amount of the active material.

The cathode may be prepared by mixing a cathode mix comprising thecathode active material, a conductive material and a binder, with apredetermined solvent such as water or NMP to prepare a slurry, andapplying the slurry to a cathode current collector, followed by dryingand pressing.

The cathode mix may optionally contain at least one selected from thegroup consisting of a viscosity controller and a filler.

The cathode current collector is generally fabricated to have athickness of 3 to 500 μm. There is no particular limit as to the cathodecurrent collector, so long as it has suitable conductivity withoutcausing adverse chemical changes in the fabricated battery. Examples ofthe cathode current collector include stainless steel, aluminum, nickel,titanium, sintered carbon, and aluminum or stainless steel which hasbeen surface-treated with carbon, nickel, titanium or silver. Ifnecessary, these current collectors may also be processed to form fineirregularities on the surface thereof so as to enhance adhesive strengthto the cathode active materials. In addition, the current collectors maybe used in various forms including films, sheets, foils, nets, porousstructures, foams and non-woven fabrics.

The conductive material is commonly added in an amount of 0.01 to 30% byweight, based on the total weight of the mixture comprising the cathodeactive material. Any conductive material may be used without particularlimitation so long as it has suitable conductivity without causingadverse chemical changes in the battery. Examples of conductivematerials include conductive materials, including graphite; carbon blacksuch as carbon black, acetylene black, Ketjen black, channel black,furnace black, lamp black and thermal black; conductive fibers such ascarbon fiber and metallic fibers; metallic powders such as carbonfluoride powders, aluminum powders and nickel powders; conductivewhiskers such as zinc oxide and potassium titanate; conductive metaloxides such as titanium oxide; and polyphenylene derivatives.

The binder is a component which enhances binding of an electrode activematerial to a conductive material and current collector. The binder iscommonly added in an amount of 1 to 30% by weight, based on the totalweight of the mixture comprising the cathode active material. Examplesof the binder include polyvinylidene, polyvinyl alcohol,carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene propylene diene terpolymer (EPDM),sulfonated EPDM, styrene butadiene rubbers, fluororubbers and variouscopolymers.

The viscosity controller controls the viscosity of the electrode mix soas to facilitate mixing of the electrode mix and application thereof tothe current collector and may be added in an amount of 30% by weight,based on the total weight of the electrode mix. Examples of theviscosity controller include, but are not limited to,carboxymethylcellulose, polyacrylic acid and polyvinylidene fluoride. Ifnecessary, the solvent may also serve as a viscosity controller.

The filler is a component optionally used to inhibit expansion of theelectrode. Any filler may be used without particular limitation so longas it does not cause adverse chemical changes in the manufacturedbattery and is a fibrous material. Examples of the filler include olefinpolymers such as polyethylene and polypropylene; and fibrous materialssuch as glass fibers and carbon fibers.

The present invention provides a lithium secondary battery comprisingthe cathode, the anode, a separator, and a lithium salt-containingnon-aqueous electrolyte.

For example, the anode is prepared by applying an anode mix comprisingan anode active material to an anode current collector, followed bydrying. The anode mix may comprise the afore-mentioned components, i.e.,the conductive material, the binder and the filler, if necessary.

The anode current collector is generally fabricated to have a thicknessof 3 to 500 μm. There is no particular limit as to the anode currentcollector, so long as it has suitable conductivity without causingadverse chemical changes in the fabricated battery. Examples of theanode current collector include copper, stainless steel, aluminum,nickel, titanium, sintered carbon, and copper or stainless steel whichhas been surface-treated with carbon, nickel, titanium or silver, andaluminum-cadmium alloys. Similar to the cathode current collector, ifnecessary, these current collectors may also be processed to form fineirregularities on the surface thereof so as to enhance adhesive strengthto the anode active materials. In addition, the current collectors maybe used in various forms including films, sheets, foils, nets, porousstructures, foams and non-woven fabrics.

Examples of the anode active material include carbon and graphitematerials such as natural graphite, artificial graphite, expandedgraphite, carbon fiber, hard carbon, carbon black, carbon nanotubes,perylene, activated carbon; metals alloyable with lithium, such as Al,Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and Ti and compoundscontaining these elements; composites of carbon and graphite materialswith a metal and a compound thereof; and lithium-containing nitrides. Ofthese, a carbon-based active material, a silicon-based active material,a tin-based active material, or a silicon-carbon-based active materialis more preferred. The material may be used alone or in combination oftwo or more thereof.

The separator is interposed between the cathode and anode. As theseparator, an insulating thin film having high ion permeability andmechanical strength is used. The separator typically has a pore diameterof 0.01 to 10 μm and a thickness of 5 to 300 μm. As the separator,sheets or non-woven fabrics made of an olefin polymer such aspolypropylene and/or glass fibers or polyethylene, which have chemicalresistance and hydrophobicity, are used. When a solid electrolyte suchas a polymer is employed as the electrolyte, the solid electrolyte mayalso serve as both the separator and electrolyte.

The lithium salt-containing, non-aqueous electrolyte is composed of anon-aqueous electrolyte and a lithium salt. As the non-aqueouselectrolyte, a non-aqueous electrolyte, solid electrolyte and inorganicsolid electrolyte may be utilized.

Examples of the non-aqueous electrolyte that can be used in the presentinvention include non-protic organic solvents such asN-methyl-2-pyrollidinone, propylene carbonate, ethylene carbonate,butylene carbonate, dimethyl carbonate, diethyl carbonate,gamma-butyrolactone, 1,2-dimethoxy ethane, tetrahydroxy Franc, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphoric acid triester, trimethoxy methane,dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate and ethylpropionate.

Examples of the organic solid electrolyte utilized in the presentinvention include polyethylene derivatives, polyethylene oxidederivatives, polypropylene oxide derivatives, phosphoric acid esterpolymers, poly agitation lysine, polyester sulfide, polyvinyl alcohols,polyvinylidene fluoride, and polymers containing ionic dissociationgroups.

Examples of the inorganic solid electrolyte include nitrides, halidesand sulfates of lithium such as Li₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH,LiSiO₄, LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH andLi₃PO₄—Li₂S—SiS₂.

The lithium salt is a material that is readily soluble in theabove-mentioned non-aqueous electrolyte and examples thereof includeLiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂,LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroboranelithium, lower aliphatic carboxylic acid lithium, lithium tetraphenylborate and imides.

Additionally, in order to improve charge/discharge characteristics andflame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol,aluminum trichloride or the like may be added to the non-aqueouselectrolyte. If necessary, in order to impart incombustibility, thenon-aqueous electrolyte may further contain halogen-containing solventssuch as carbon tetrachloride and ethylene trifluoride. Further, in orderto improve high-temperature storage characteristics, the non-aqueouselectrolyte may further contain carbon dioxide gas or the like and mayfurther contain fluoro-ethylene carbonate (FEC), propene sulfone (PRS),fluoro-propylene carbonate (FPC) and the like.

The lithium secondary batteries according to the present invention maybe used as unit batteries of battery modules, which are power sources ofmedium and large devices requiring high-temperature stability, longcycle and superior rate characteristics.

Preferably, examples of medium and large devices include power toolspowered by battery-driven motors; electric vehicles including electricvehicles (EVs), hybrid electric vehicles (HEVs) and plug-in hybridelectric vehicles (PHEVs); electric two-wheeled vehicles includingelectric bikes (E-bikes), electric scooters (E-scooter); electric golfcarts and the like.

Accordingly, the present invention provides a middle or large batterypack comprising the secondary battery as a unit battery. The generalstructure and production method of the middle or large battery pack areknown in the art and a detailed explanation thereof is omitted herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a graph showing discharge profiles at 5^(th) and 25^(th)cycles for the battery of Example 1 and the battery of ComparativeExample 1 in the Experimental Example 2.

BEST MODE

Now, the present invention will be described in more detail withreference to the following examples. These examples are provided only toillustrate the present invention and should not be construed as limitingthe scope and spirit of the present invention.

Example 1

A mixed transition metal precursor having a composition ofNi:Mn:Co=0.53:0.27:0.2 (molar ratio) was prepared by a co-precipitationmethod known in the art, and lithium hydroxide and Li₃PO₄ were added tothe mixed transition metal precursor such that conditions of x=0.01,y=0.02, z=0.0 and b=0 in Formula 1 were satisfied, followed by baking ina furnace to synthesize a cathode active material.

The synthesized cathode active material was mixed with NMP such that aratio of active material:conductive material:binder became 95:2.5:2.5(weight ratio), and the mixture was coated on an Al foil with athickness of 20 μm to produce a cathode. The cathode was pressed suchthat an inner pore ratio was 25% to fabricate a coin-type battery. ALi-metal foil was used as an anode and a solution of 1M LiPF₆ in acarbonate mixed solvent (EC:DMC:DEC=1:2:1, volume ratio) was used as anelectrolyte.

Example 2

A battery was fabricated in the same manner as in Example 1 except thata cathode active material having a composition of x=0.03 wassynthesized.

Comparative Example 1

A battery was fabricated in the same manner as in Example 1 except thata cathode active material having a composition of x=0 was synthesized.

Example 3

A battery was fabricated in the same manner as in Example 1 except thata cathode active material having a composition of x=0.05 wassynthesized.

Example 4

A battery was fabricated in the same manner as in Example 1 except thata cathode active material having a composition of x=0.005 wassynthesized.

Example 5

A battery was fabricated in the same manner as in Example 1 except thata cathode active material was synthesized by adding aluminum hydroxidesuch that a part (z=0.02) of the transition metal was substituted by Al,followed by baking.

Comparative Example 2

A battery was fabricated in the same manner as in Example 1 except thata cathode active material having a composition of x=0 was synthesized,and mixed with NMP such that active material: lithium phosphate:conductive material: binder was 90:5:2.5:2.5 (weight ratio), and thecathode active material was coated on an Al foil with a thickness of 20μm to fabricate a cathode.

Experimental Example 1

The batteries fabricated in Examples 1 to 4 and Comparative Examples 1and 2 were charged and discharged at 0.1 C, capacities thereof weremeasured, and deterioration in capacity with cycles was measured undercharge and discharge conditions of 0.5 C. The results thus obtained areshown in the following Table 1.

TABLE 1 Discharge capacity 30^(th) cycle capacity/1^(st) (mAh/g) cyclecapacity (%) Ex. 1 166 97.4 Comp. Ex. 1 165 93.0 Ex. 2 166 97.8 Ex. 3160 98.2 Ex. 4 168 97.3 Ex. 5 165 97.5 Comp. Ex. 2 153 78

As can be seen from Table 1 above, although the batteries of Examples 1to 4 contained Li₃PO₄ not directly contributing to charge and discharge,they did not exhibit a great difference in initial capacity. As thecontent of Li₃PO₄ increased, the capacity thereof slightly decreased,but was not significant.

On the other hand, the batteries (Examples 1 to 5) using a cathodeactive material containing Li₃PO₄ exhibited a considerably low capacitydeterioration with an increase in cycles, as compared to the batteries(Comparative Examples 1 to 2) using a cathode active material containingno Li₃PO₄. Specifically, for the 30^(th) cycle capacity to the 1^(st)cycle capacity, the batteries of Examples 1 to 5 exhibited at least 4%or higher capacity, as compared to the battery of Comparative Example 1.This difference reached several tens of % at 300 cycles or more, and asdescribed above, batteries for vehicles are charged 1000 cycles or moreand under these conditions, the difference increased.

Furthermore, the battery of Comparative Example 2 exhibited 15% or morelower cycle properties, as compared to the batteries of Examples 1 to 5.This is due to the fact that battery capacity is decreased, cyclecharacteristics are rapidly deteriorated and electrical conductivity waslowered by separately adding Li₃PO₄ to the cathode active material. Thebattery of Comparative Example 2 was greatly distinguished withbatteries of Examples 1 to 5 fabricated using a cathode active materialsynthesized by mixing other precursors with Li₃PO₄, followed by baking.

Experimental Example 2

The batteries fabricated in Example 1 and Comparative Example 1 werecharged and discharged 5 and 25 cycles at 0.5 C, and discharge profilesat these cycles are shown in FIG. 1.

As can be seen from FIG. 1, the battery of Example 1 exhibiteddeterioration at the end stage of discharge, and in particular, aremarkable decrease in voltage drop, as compared to the battery ofComparative Example 1. This means that deterioration is decreased due tostructural change of the cathode. Such deterioration at the end stage ofdischarge is the most important factor that rapidly deteriorates thepower of batteries for electric vehicles or hybrid electric vehicles andthe factor is more important than a decreased capacity that can bemeasured in general batteries.

In this regard, the cathode active material of the present invention canconsiderably reduce deterioration at the end stage of discharge. As canbe seen from FIG. 1, such a phenomenon becomes serious, as the number ofcycles increases. That is, the deterioration difference at the end stageof discharge at the 25^(th) cycle is greater than that at the 5^(th)cycle.

Batteries for vehicles require 3600 cycles or more of charge anddischarge although they are charged and discharged only once a day underproduct guarantee conditions of 10 years or longer, thus making thisdifference considerably great. Accordingly, small difference in smallconventional batteries further increases in batteries for vehicles, anddifference in cycle properties, variation in charge and dischargeprofiles and the like are more important than the small difference incapacity.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

INDUSTRIAL APPLICABILITY

As apparent from the afore-going, the cathode according to the presentinvention can improve lifespan properties based on a specific elementcomposition, and in particular, is thus preferably useful for devicesrequiring use for a long period of time due to superior cycleproperties.

The invention claimed is:
 1. A cathode for secondary batteriescomprising a compound having a transition metal layer containing lithiumas at least one compound selected from the following formula 1:(1−x)Li(Li_(y)M_(1-y-z)Ma_(z))O_(2-b)A_(b)*xLi₃PO₄  formula (1) whereinM has a composition of the following formula 2:Ni_(a)Mn_(b)Co_(c)  formula (2), Ma is a metal or non-metal elementstable for a six-coordination structure; A is at least one selected fromthe group consisting of halogen, sulfur, chalcogenide compounds andnitrogen;0<x<0.1;0<y<0.3;0≦z<0.2;0≦b<0.1;0.10<a<0.85;0.10<b<0.85;0<c<0.5;andb>c.
 2. The cathode for secondary batteries according to claim 1,wherein x satisfies the condition of 0<x≦0.05.
 3. The cathode forsecondary batteries according to claim 1, wherein y satisfies thecondition of 0.01≦y≦0.2.
 4. A lithium secondary battery comprising thecathode according to claim
 1. 5. A middle or large battery packcomprising the lithium secondary battery according to claim 4 as a unitbattery.